Non-reactive coatings for inertization

ABSTRACT

A method for the selective or complete rendering inert of substrates, workpieces, and components of the apparatus in the chemical deposition of metals by depositing nonreactive coatings thereon. A coating which is nonreactive with respect to the subsequent plating is electrolytically deposited on the sites of the substrate which are not to be plated from a solution containing an activator and an electrolyte. The inert, nonreactive coating is removable by means of dilute acids or alkaline media, without manual effort, after the subsequent chemical plating of the substrates, workpieces, or unit parts.

FIELD OF THE INVENTION

The present invention relates generally to a method for the selective or complete inertization of substrates, workpieces, and unit parts by application of nonreactive coatings to those surfaces that are not to be plated during a metal deposition process.

BACKGROUND OF THE INVENTION

In addition to the electrolytic method for the plating of basic materials, so-called currentless coating methods, i.e., electroless plating, have been known for a long time. “Electroless” or “chemical” plating is understood to be a chemical surface refinement of almost all metals and many nonconductive materials. It essentially differs in its chemical and mechanical features from galvanically applied metal coatings. Thus the electroless plating with metals is based on an autocatalytic process. With such plating methods, a reaction takes place on catalytically active surfaces. The metal ions contained in the deposition bath, i.e., electrolytes, are reduced to elementary metal, whereas at least one reducing agent contained in the electrolyte is oxidized.

For the selective coating of substrates, workpieces, or unit parts with metals, therefore, it is necessary to cover the areas which are not to be plated with noncatalytically active or nonelectrically conductive material, rendering these areas inert during the plating process. From the methods for chemical nickel plating already known in the state of the art, one can deduce that individual components of the process apparatus to be used, such as the container for the deposition bath, the frames for the substrates to be coated, etc., are passivated before use in order to avoid an undesired coating with nickel and, moreover, an unnecessary consumption of the electrolyte. To this end, the unit parts made mostly of stainless steel, such as tanks, pumps, heating elements, or stirrers, etc., are usually passivated by treatment with concentrated acid. In addition, an anodic potential is applied to the unit parts in order to maintain their passive state. In spite of these measures, however, a repetition of the treatment with nitric acid at regular intervals is necessary. This represents a considerable investment in cost and time for the coating operation, on the one hand, since the production must be interrupted and since nitric acid is a hazardous substance; on the other hand, expensive safety measures for the protection of the environment and personnel must be taken.

Moreover, methods are known in the state of the art, wherein such a covering of areas of a workpiece which are not to be plated is usually done by using lacquers of plastic materials. These methods for the selective covering or passivation, however, also represent considerable process-related expense. In addition, any subsequent removal of the passivation layer and the disposal of the barely recyclable remaining materials leads to high disposal costs.

SUMMARY OF THE INVENTION

Among the several aspects of the invention is to provide a method for the selective or complete inertization of substrates, workpieces, and unit parts during a metal deposition process.

Briefly, therefore, the invention is directed to a method for depositing a nonreactive coating which is inert to metal plating onto a surface of a substrate which is not to be plated in a subsequent metal deposition process performed on the substrate, the method comprising activating the surface for electrolytic deposition of the nonreactive coating and electrolytically depositing the nonreactive coating onto the surface.

The invention is also directed to an electrolyte for depositing a nonreactive coating which is inert to metal plating onto a surface of a substrate which is not to be plated in a subsequent metal deposition process performed on the substrate, the electrolyte comprising a fluoride activator and a source of chromium ions for the nonreactive coating.

Other objects and features will be in part apparent and in part pointed out hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

This application claims priority of European application no. 03018449.3, filed Aug. 14, 2003, the entire disclosure of which is explicitly incorporated by reference.

The invention involves the prevention of metal plating on some or all of a substrate's surface exposed to an electroless metal deposition process. This is accomplished, in accordance with the invention, by a method for the selective or complete inertization of substrates, workpieces, or unit parts in the metal plating process, which comprises the electrolytic deposition of a firmly affixed nonreactive coating on areas which are not to be plated. Here, “unit part” refers to a component of a metal deposition apparatus, specifically one that is not to be plated in a subsequent metal deposition process.

In the method in accordance with the invention, the disadvantages known in the state of the art in the selective plating by the electrolytic deposition of an inertization layer are eliminated. The average coating times are shortened considerably by the simplified execution of the inertization, which minimizes disposal costs, and in this way, attains a considerably more economical process. This is predominantly based on the advantageous composition of the electrolyte used for the inertization. Surprisingly, it turns out that in particular, by the use of electrolytes with chromium, it is possible to electrolytically deposit an inertization layer on the area of a substrate which is not to be plated that exhibits a completely inert behavior in a process solution for the chemical plating of typical metals, such as copper, nickel, zinc, tin, lead, bismuth, palladium, silver, gold, platinum, etc. Even when there is electrical contact between the inert surface and the surfaces that are subject to the autocatalytic reaction of the electroless deposition of metals (e.g., the noncoated areas of a workpiece or unit components, etc.), there is no activation and thus also, no undesired coating of the inert areas.

A selective or complete inert coating, in accordance with the invention, can advantageously be applied on the most varied substrates. In this way, a coating of workpieces or unit components made of metals, metal compounds, alloys, or plastics also is possible, wherein the plastics were plated in the past according to other methods. This plating can already be carried out partially or selectively, and can be performed by immersing the surface in the electrolyte. The coated areas are then resistant to subsequent electroless metal plating.

Another advantage of the method of the invention is that the deposition process of metals can also be designed more economically by a passivation of the unit components, in accordance with the invention. Thus, by rendering inert the container with the electrolytes for electroless plating, the average service life of the electrolytic bath can be considerably and advantageously prolonged and the execution of plating can be simplified, since, among other things, the application of an anodic potential to maintain the passive state is omitted. In addition, safety measures can be limited and costs are correspondingly minimized. Advantageously, unit components made of metal and/or metal compounds, preferably steel or, more preferably, stainless steel, such as the containers for the deposition baths or the racks for the workpieces to be coated, are rendered inert with a nonreactive surface. Such inertization makes possible the use of far cheaper materials, such as conventional steels, instead of very expensive stainless steels.

Depending on which substrates are to be selectively rendered inert or plated or which additional properties the metal layers are to have, the substrates are activated before being rendered inert. This is particularly necessary with nonconductive substrates. The activation of the substrates can take place in a traditional way. Advantageously, the activation takes place simultaneously with the inertization step by employing an electrolyte comprising an activator. In this context, it has proved advantageous to mix electrolytes prepared for inertization with an activator of a fluoride- and/or fluorine-containing solution in order to attain a sufficient activation of the material's surface, particularly stainless steel surfaces, and thus to ensure an adhesive coat with the inertization step.

The process of inertization takes place by the electrolytic deposition of a nonreactive coating from a suitable electrolyte, preferably comprising chromium, with the application of a cathode current to produce a chromium-based nonreactive coating. Usually, to deposit coatings with chromium—particularly in so-called chromium baths—at temperatures in the range of 40-70° C., current densities in the range of 10-50 A/dm² are required. Since this requires considerable total current, which is not suitable in every case or for every inertization process, it has proved advantageous to deposit the inertization layer in a small current density range. Advantageously and in accordance with this invention, an electrolyte is used that makes possible an adhesive covering with a nonreactive coating below a current density of 10 A/dm², preferably below 5 A/dm², at low temperatures. Preferably, the process is conducted below about 40° C., more preferably in the range of about 15-40° C. These can be so-called “cold chromium baths.” According to this invention, chromium baths based on chromium compounds prepared using various methods can be used, such as baths based on trivalent chromium (e.g., TRICROLYTE®, commercially available from Enthone Inc.).

Subsequent electroless plating of the areas of the substrate which are not rendered inert can take place in a traditional manner. Advantageously, an electrolyte is used in the context of this invention that is also used with conventional electroless plating.

Moreover, the selectively applied nonreactive coating can be removed after the electroless metal plating without manual effort and in a simple manner. This can be done by contacting the nonreactive coating with a dilute acid, or if the substrate is endangered by such an acid, by contacting the nonreactive coating with an alkali solution while applying an anodic polarization until the coating is completely removed.

The following example further illustrates the invention:

EXAMPLE

In accordance with the invention, an electrolyte for the production of a selective, nonreactive coating on a substrate in the metal deposition process comprises: (1) an activation solution with a fluoride and/or fluoride compound, and (2) one or more sources for chromium ions, such as chromium-based salts, preferably chromic acid or trivalent chromium compounds to provide precursor ions for depositions of the nonreactive coating.

An electrolyte with the following composition was created:

-   200-300 g/L chromium(VI) oxide; -   1-3 g/L concentrated sulfuric acid; -   0.02-0.3 g/L fluoride; and -   0.2-3 g/L methanesulfonic acid or methanesulfonic acid derivative.

A substrate was exposed to the electrolyte at a bath temperature of 20° C. and with an applied current density of 3 A/dm², which deposited smooth, thick layers without layer thickness limitation onto the substrate.

Application examples for the method and electrolytes of the present invention, in accordance with the invention, are the coating of stainless steels with chromium layers, which are inert with reference to subsequent plating. The use of a pretreatment nickel bath, which is common according to the state of the art, can be omitted. A direct selective or complete coating with an inert chromium layer is possible.

Another favorable use of the method and electrolytes of the invention is the coating of the interior surface of containers for chemical plating baths to prevent a plating of these surfaces. Advantageously, the container can be filled with an electrolyte in accordance with the invention to the maximum coating level and a corresponding voltage can subsequently be applied to the chemical plating bath container. It is particularly advantageous here that the use of far more expensive stainless steels for containers, pumps, heatings, stirrers, etc., can be dispensed with, since the surfaces coated by means of the electrolyte in accordance with the invention are inert with respect to the chemical plating baths.

Another application example is the interior tin plating of bath fixtures after chrome plating its exterior in the method of the invention or with the electrolyte according to the invention. Moreover, it is possible to nickel plate ball valves by the selective rendering inert of sites exposed to aggressive media, whereas the surfaces which exhibit abrasive wear can be chromed.

In another use, heat exchanger elements made of copper or copper alloys for gas therms are selectively coated with an inert chromium layer, wherein the cavities are furnished with a chemical nickel coat for corrosion protection reasons. The chromed exterior areas are resistant to hot combustion gases, whereas the water-conducting interior is resistant to corrosion from water or other heat-carrying media.

When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 

1. A method for depositing a nonreactive coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: activating the surface for electrolytic deposition of the nonreactive coating; and electrolytically depositing the nonreactive coating onto the surface.
 2. The method of claim 1 wherein the nonreactive coating is inert to plating of a metal selected from the group consisting of copper, nickel, zinc, tin, lead, bismuth, palladium, silver, gold, and platinum, and combinations thereof.
 3. The method claim 1 wherein said activating comprises contacting the surface with a fluoride-containing solution.
 4. The method of claim 1 wherein the surface is a material selected from the group consisting of metals, metal compounds, alloys, plastics, and combinations thereof.
 5. The method of claim 1 wherein said electrolytically depositing the nonreactive coating comprises depositing from a chromium-containing electrolyte.
 6. The method of claim 1 wherein said electrolytically depositing the nonreactive coating comprises application of a cathode current.
 7. The method of claim 1 wherein the electrolytic deposition of the nonreactive coating comprises application of a cathode current having a current density below about 10 A/dm².
 8. The method of claim 1 wherein the electrolytic deposition of the nonreactive coating comprises application of a cathode current having a current density below about 5 A/dm².
 9. The method of claim 1 wherein the electrolytic deposition of the nonreactive coating is performed at a temperature below about 40° C.
 10. The method of claim 1 comprising electroless metal deposition of metal onto the surface after electrolytically depositing the nonreactive coating, wherein the nonreactive coating is not plated during the electroless metal deposition.
 11. The method of claim 10 comprising removing the nonreactive coating after the electroless metal deposition.
 12. The method of claim 10 comprising removing the nonreactive coating after the electroless metal deposition by contacting the nonreactive coating with an acid.
 13. The method of claim 10 comprising removing the nonreactive coating after the electroless metal deposition by contacting the nonreactive coating with an alkaline solution and applying anodic polarization.
 14. The method of claim 1 wherein the surface is of a component of an electroless metal deposition apparatus.
 15. The method of claim 1 wherein the activating the surface and the electrolytically depositing the nonreactive coating are performed in a single operation employing an electrolyte containing an activator.
 16. The method of claim 1 wherein the activating the surface and the electrolytically depositing the nonreactive coating are performed in a single operation employing an electrolyte containing a fluoride activator and a source of chromium ions for the nonreactive coating; the process further comprising: electrolessly depositing metal onto the surface after electrolytically depositing the nonreactive coating, wherein the nonreactive coating is not plated during the electroless metal deposition; and removing the nonreactive coating.
 17. A method for depositing a nonreactive coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: immersing the surface in an electrolyte comprising an activator for electrolytic deposition of the nonreactive coating and precursor ions for the nonreactive coating; and applying an external source of electrons to electrolytically deposit the nonreactive coating onto the surface.
 18. The method of claim 17 comprising: electrolessly depositing metal onto the surface after electrolytically depositing the nonreactive coating, wherein the nonreactive coating is not plated during the electroless metal deposition.
 19. A method for depositing a chromium-based nonreactive coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: immersing the surface in an electrolyte comprising a fluoride activator and chromium ions for the nonreactive coating; and applying an external source of electrons to electrolytically deposit the chromium-based nonreactive coating onto the surface.
 20. The method of claim 19 wherein the electrolyte comprises between about 0.02 and about 0.3 g/L fluoride and between about 200 and about 300 g/L chromium oxide.
 21. The method of claim 19 comprising: electrolessly depositing metal onto the surface after electrolytically depositing the chromium-based nonreactive coating, wherein the chromium-based nonreactive coating is not plated during the electroless metal deposition.
 22. A method for depositing a chromium-based nonreactive coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition to which the surface is exposed, the method comprising: immersing the surface in an electrolyte comprising between about 0.02 and about 0.3 g/L fluoride, between about 200 and about 300 g/L chromium oxide, between about 1 and about 3 g/L sulfuric acid, and between about 0.2 and 3 g/L methanesulfonic acid or a derivative thereof; and applying an external source of electrons to electrolytically deposit the chromium-based nonreactive coating onto the surface.
 23. A method for depositing a nonreactive coating which is inert to metal plating onto a surface of a component of an electroless metal deposition apparatus, the method comprising: immersing the surface in an electrolyte comprising an activator for electrolytic deposition of the nonreactive coating and precursor ions for the nonreactive coating; and applying an external source of electrons to electrolytically deposit the nonreactive coating onto the surface to thereby render the surface resistant to electroless metal plating.
 24. A method for depositing a chromium-based nonreactive coating which is inert to metal plating onto a surface of a component of an electroless metal deposition apparatus, the method comprising: immersing the surface in an electrolyte comprising a fluoride activator and chromium ions; and applying an external source of electrons to electrolytically deposit the chromium-based nonreactive coating onto the surface to thereby render the surface resistant to electroless metal plating.
 25. A method for depositing a chromium-based nonreactive coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: immersing the surface in an electrolyte comprising a fluoride activator and chromium ions; applying an external source of electrons to electrolytically deposit the chromium-based nonreactive coating onto the surface; electrolessly depositing a metal selected from the group consisting of copper, nickel, zinc, tin, lead, bismuth, palladium, silver, gold, and platinum, and combinations thereof onto the surface after electrolytically depositing the chromium-based nonreactive coating, wherein the chromium-based nonreactive coating is not plated during the electroless metal deposition; and removing the chromium-based nonreactive coating.
 26. The method of claim 25 wherein the electrolyte comprises between about 0.02 and about 0.3 g/L fluoride and between about 200 and about 300 g/L chromium oxide.
 27. The method of claim 25 wherein the electrolyte comprises between about 0.02 and about 0.3 g/L fluoride, between about 200 and about 300 g/L chromium oxide, between about 1 and about 3 g/L sulfuric acid, and between about 0.2 and 3 g/L methanesulfonic acid or a derivative thereof.
 28. An electrolyte for depositing a nonreactive coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the electrolyte comprising: a fluoride activator; and a source of chromium ions for the nonreactive coating.
 29. The electrolyte of claim 28 comprising between about 0.02 and about 0.3 g/L fluoride, and between about 200 and about 300 g/L chromium oxide as the source of chromium ions.
 30. The electrolyte of claim 28 comprising between about 0.02 and about 0.3 g/L fluoride, between about 200 and about 300 g/L chromium oxide as the source of chromium ions, between about 1 and about 3 g/L sulfuric acid, and between about 0.2 and 3 g/L methanesulfonic acid or a derivative thereof.
 31. A method for depositing a chromium coating which is inert to metal plating onto a surface which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: immersing the surface which is not to be plated in the subsequent metal deposition process in an electrolyte comprising a source of chromium ions; and applying a cathodic current to the surface with a current density below about 10 A/dm² to electrolytically deposit the chromium coating which is inert to metal plating thereon.
 32. The method of claim 31 wherein the electrolytic deposition of the nonreactive coating is performed at a temperature below about 40° C.
 33. The method of claim 31 wherein the source of chromium ions is a chromium oxide.
 34. The method of claim 31 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating onto a substrate.
 35. The method of claim 31 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating of a metal selected from the group consisting of copper, nickel, zinc, tin, lead, bismuth, palladium, silver, gold, and platinum, and combinations thereof onto a substrate.
 36. A method for depositing a nonreactive coating which is inert to metal plating onto a surface of a component of a metal deposition apparatus which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: activating the surface of the component of the metal deposition apparatus for electrolytic deposition of the nonreactive coating onto the surface of the component of the metal deposition apparatus; and electrolytically depositing the nonreactive coating onto the surface of the component of the metal deposition apparatus.
 37. The method of claim 36 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating onto a substrate.
 38. The method of claim 36 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating of nickel onto a substrate.
 39. The method of claim 36 wherein said activating comprises contacting the surface with a fluoride-containing solution.
 40. The method of claim 39 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating onto a substrate.
 41. The method of claim 39 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating of nickel onto a substrate.
 42. The method of claim 36 wherein the electrolytically depositing the nonreactive coating comprises: immersing the surface of the component of the metal deposition apparatus which is not to be plated in the subsequent metal deposition process into an electrolyte comprising a source of chromium ions; and applying an external source of electrons to electrolytically deposit a chromium coating onto the surface of the component of the metal deposition apparatus which is not to be plated in the subsequent metal deposition process.
 43. The method of claim 42 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating onto a substrate.
 44. The method of claim 42 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating of nickel onto a substrate.
 45. The method of claim 39 wherein the electrolytically depositing the nonreactive coating comprises: immersing the surface of the component of the metal deposition apparatus which is not to be plated in the subsequent metal deposition process an electrolyte comprising a source of chromium ions; and applying an external source of electrons to electrolytically deposit a chromium coating onto the surface of the component of the metal deposition apparatus which is not to be plated in the subsequent metal deposition process.
 46. The method of claim 45 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating onto a substrate.
 47. The method of claim 45 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating of nickel onto a substrate.
 48. The method of claim 36 wherein the component of a metal deposition apparatus which is not to be plated in a subsequent metal deposition process is a stainless steel electroless metal deposition tank.
 49. The method of claim 38 wherein the component of a metal deposition apparatus which is not to be plated in a subsequent metal deposition process is a stainless steel electroless metal deposition tank.
 50. A method for depositing a nonreactive coating which is inert to metal plating onto a surface of a component of a metal deposition apparatus which is not to be plated in a subsequent metal deposition process to which the surface is exposed, the method comprising: immersing the surface of the component of the metal deposition apparatus which is not to be plated in the subsequent metal deposition process an electrolyte comprising a source of chromium ions; and applying a cathodic current to the surface with a current density below about 10 A/dm² to electrolytically deposit the chromium coating thereon.
 51. The method of claim 50 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating onto a substrate.
 52. The method of claim 50 further comprising exposing the surface which is not to be plated in the subsequent metal deposition process to the subsequent metal deposition process, wherein the subsequent metal deposition process comprises electroless plating of nickel onto a substrate. 